.Defenses
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  Nutritional content
 

Defenses of nudibranchs and their relatives include nutritional content, considered in this section, and
CAMOUFLAGE (CRYPSIS)
,
FAST CRAWLING & SWIMMING,
MUCOUS COATINGS,
CERATAL AUTOTOMY,
SPICULES
,
NEMATOCYSTS,
VACUOLATED SKIN WITH PROTECTIVE SPINDLES
,
ACID SECRETIONS,
INK & OPALINE SECRETIONS,
SECONDARY METABOLITES
,
ALARM PHEROMONES,
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

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Research study 1
 

graph showing % organic content of different species of gastropodsAn interesting idea relating to defenses of nudibranchs is that, like crinoids and other calcareous invertebrates, there is a large component of non-nutritional matter in their bodies.  For example, a comparison at Bamfield Marine Sciences Centre, British Columbia of soft tissues of 3 species of shelled orthogastropods with 7 species of nudibranchs (4 spicule-containing species and 3 non-spicule-containing species) shows a 2-3-fold lesser organic content in the histogram showing ingest of artificial pellets containing different amounts of organic mass by crabs Cancer productusnudibranchs (see illustration on Left). Thus, all things being equal, a predator will obtain correspondingly less nutrients per bite of soft tissue if it selects a nudibranch over a shelled orthogastropod as prey. 

In support of this idea, lower organic content has a significant negative effect on feeding on artificial diets by predatory crabs Cancer productus in no-choice tests in the laboratory (see graph on Right).  The graph indicates a significant depression in feeding by the crabs on artificial diets (powdered squid set in alginate gel) containing 12% dry-mass organic content versus ones containing 22% dry-mass organic content.  Simply put, the strategy being demonstrated is one of a prey being “just not worth eating”.  The author concludes that reduced organic content per bite (equated in the paper with reduced nutritional quality) may offer anti-predator deterrence to nudibranchs independent of any chemical defenses that they may have.  Penney 2002 Oecologia 132: 411.

NOTE  the graph also shows that with the exception possibly of foot and viscera, whether a nudibranch has spicules or not appears to have little effect on its overall organic content.  Moreover, the areas of non-significant differences, “margin” and “center”, are the ones likely to be encountered first by a predator.  Spicules as defense for nudibranchs are considered elsewhere in this section of the ODYSSEY: NUDIBRANCH DEFENSES: SPICULES

NOTE  many nudibranchs contain secondary metabolites that may function in defense, as shown in: NUDIBRANCH DEFENSES: SECONDARY METABOLITES

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